[clang] Place strdup in the unix.cstring.NullArg list (PR #106387)

[Balazs Benics via cfe-commits]

steakhal wrote:

In the gcc's stadlib, strdup is already marked as `__attribute__ ((__nonnull__ (1))`, which is checked by `core.NonNullParamChecker`. The bug is [caught](https://godbolt.org/z/3fbrx4jT1), as of today.

I don't think there is anything to be done here.

https://github.com/llvm/llvm-project/pull/106387